1. Field of the Invention
The present invention relates to a manner of control of the illumination system in a lithographic apparatus.
2. Discussion of Related Art
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The illumination system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam of radiation, and such elements may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively. Further, the lithographic apparatus may be of a type having two or more mask tables and/or two or more substrate tables. In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more stages while one or more other stages are being used for exposures. Twin stage lithographic apparatuses are described in International Patent Applications WO 98/28665 and WO 98/40791, for example.
Lithographic projection apparatuses can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies which are successively irradiated via the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed v at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
In a lithographic apparatus, the size of features that can be imaged onto the wafer is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices and hence higher operating speeds, it is desirable to be able to image smaller features. Whilst most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use higher frequency (energy) radiation, e.g. EUV or X-rays, or particle beams, e.g. electrons or ions, as the illumination radiation in lithographic apparatuses.
However, the glass or quartz plates on which a conventional reticle pattern is defined are generally not transparent to some of these forms of illumination radiation. As an alternative in the case of charged-particle lithography, for example, the reticle is formed of a material, e.g. metal, that is opaque to the form of radiation used and in which apertures are cut to define the reticle pattern. To avoid the need to provide obscuring support arms to opaque islands in the pattern, the reticle pattern is divided into a plurality of sub-patterns separated by supporting struts. The complete pattern is correctly imaged on the wafer by introducing successive shifts in the illumination beam after it has passed through each sub-pattern. This type of arrangement is sometimes referred to as a xe2x80x9cstrutted maskxe2x80x9d and an example is disclosed in U.S. Pat. No. 5,079,112.
General information with regard to the use of electron beams in lithography can be gleaned, for example, from U.S. Pat. No. 5,260,151.
The image quality of lithography systems using strutted masks can be reduced by distortion of the masks due to heating and, in charged particle apparatuses, by the presence of randomly scattered particles.
An object of the present invention is to avoid or reduce the effect of such factors on the image quality of lithographic projection apparatus.
The invention relates to the control of the illumination system in a lithographic projection apparatus that has a radiation system having a radiation source and an illumination system for supplying a projection beam of radiation; a first movable object table provided with a mask holder for holding a mask; a second movable object table provided with a substrate holder for holding a substrate; and a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate.
According to the present invention there is provided a lithographic projection apparatus for scan-imaging of a mask pattern in a mask onto a substrate provided with a radiation sensitive layer, the mask having at least one transmissive region bounded at least in the scan direction by opaque regions. The apparatus include a radiation system comprising a radiation source and an illumination system for generating an illumination beam and controlling said illumination beam to scan said transmissive region between said opaque regions; a first movable object table provided with a mask holder for holding a mask; a second movable object table provided with a substrate holder for holding a substrate; and a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate: The illumination system is embodied to begin a scan by generating a relatively narrow beam adjacent one of said opaque regions, to increase the width of the beam whilst it remains adjacent the opaque region, and to scan the beam towards the other of the opaque regions, thereby to provide a substantially uniform exposure on the substrate.
The term xe2x80x9ctransmissive regionxe2x80x9d is intended to refer to a region of the mask which is at least substantially transparent to the illumination radiation used.
With the present invention it is possible to avoid problems that occur when the illumination beam impinges on an opaque region (strut or skirt)-e.g. distortion of the reticle caused by excessive heating and scattering of particlesxe2x80x94whilst at the same time ensuring that a correct exposure (dose) is given at the extremities of each transmissive region (sub-field).
The present invention provides a further advantage in that it is possible to scan the lengths of elongate sub-fields xe2x80x9celectronicallyxe2x80x9d and to scan between sub-fields xe2x80x9cmechanicallyxe2x80x9d, as will be explained in greater detail in the embodiments below.
According to a yet further aspect of the invention there is provided a method of manufacturing a device using a lithographic projection apparatus having a radiation system comprising a radiation source and an illumination system for generating an illumination beam; a first object table provided with a mask holder for holding a mask; a second object table provided with a substrate holder for holding a substrate; and a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate provided with a radiation sensitive layer. The method scan-imaging a mask pattern in the mask onto the target portion of the substrate, the mask having at least one transmissive region bounded at least in the scan direction by opaque regions. The imaging step includes at the beginning of a scan, generating a relatively narrow beam adjacent one of said opaque regions, increasing the width of said beam whilst it remains adjacent said opaque region, and scanning said beam towards the other of said opaque regions, thereby to provide a substantially uniform exposure on the substrate.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallisation, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.
In the present document, the terms illumination radiation and illumination beam are used to encompass all types of electromagnetic radiation or particle flux, including, but not limited to, ultraviolet radiation, EUV, X-rays, electrons and ions.